This is most probably WAI, following the persistent per-device audio selection method -  issue 308143 , being introduced in M51. 

If headphone was unselected during the last user login, then this should be a WAI.
Assign to Jenny since she knows the exact logic.

This is seen on Blaze device as well.

Did you follow 1), 2) and 3) with no manually selecting audio output in between?

If yes - then the new model(per selected device) should not be taking effect and the old model(per type) should apply with Headphone output presented.

If no - then anything you selected before plugging should persist.

I dont think anything(state of audio nodes) before the autoupdate has to do with after-autoupdate state.

Have the headphone been plugged in before and not selected as the active device? If yes, this is expected behavior. 

If you do a power wash, then follows all repro steps, the headphone should be selected as active when it plugs in.

#5: correct.

I have seen the following behavior on Blaze Device:
1. It was auto-updated from M50 build to M51 build 51.0.2686.0
2. i Connected the head phones before i logged in to the device.
3. Logged in and started playing the video.
4. Sound out was via the device speakers instead of Headset.  I have to manually select the headset in the settings as sound output.

This never happened before. The sound usually used to play in the headset with out selecting anything in the settings before when the head set is connected before playing any video or game.

Is this something new which was introduced in M51 builds ?

Step #1, before you connect the headphone, check which device is active.
Step #2, after connect the headphone, before login, check which device is active.

Unable to reproduce the issue on Chrome 51.0.2687.0/8103.0.0 - Candy. 
Upgraded the Candy device from 50.0.2661.32/7978.18.0 and headphone(Logitech) is selected automatically when connecting to the device.

 Issue 610395  has been merged into this issue.

I've upgraded my acer c740 to 51 and i have this issue, too.

QA, has anyone can reproduce this issue with a clean device?

There are several reports in CBC after the update to 51.

2 Asus Chromeboxes
https://productforums.google.com/forum/#!topic/chromebook-central/wMKvlVdF-Us

Two more threads

https://productforums.google.com/forum/#!topic/chromebook-central/Czhao08-fIQ
https://productforums.google.com/forum/#!topic/chromebook-central/rjZI9TvQ6TQ

#CBC-RS/TC-watchlist

I tested the current prod stable build 8172.47.0/51.0.2704.79 and followed steps in #8 with a blaze device after recovery to this build and also power-washed.

- Connected the audio jack headphones before sign-in (i observed the Headphones audio node active in Audio settings)
- signed in and started playing video - audio came through the headphones
- unplugged and plugged back several times - all worked out OK - onboard speaker on unplug, and headphones on plug back.

The threads in c#14 are regarding  issue 617273 .

From #16, it sounds like working fine. Is this one still an issue?

Not able to reproduce this issue with M53 build 8530.57.0_samus and M52 build 8350.68.0_enguarde

Im on 8350.68.0 (Official Build) stable-channel auron_paine 52.0.2743.116 (Offizieller Build) (64-Bit) and i still have this issue. I was forced to powerwash my device, so this bug reappear. When i manually select the headphone, then it works.

Hi andreash8577,

Was your audio headphones plugged while powerwashing?
Would you mind to powerwash the DUT again(with your external audio unplugged), and if the issue comes up again, file a feedback report with Shift+Alt+i, and submit the feedback form. 

Hi kalin,

yep it seems my headphones was plugged in. I did it again without and now it works after powerwash. Thanks

I see,
So what happens is that after powerwash, the active node is set to onboard speaker, and the plugged(at the time of powerwash first boot) headphones jack is registered as inactive. I guess this corner use case is not covered. Still it is valid and uncovers this bug.

Re-opening

The intended behavior is to have the headphones jack(or any other) audio node be set as the active one if it is presented at time of first boot after recovery or powerwash.

It seems the  issue 622045  is representing what is happening here.